Antenex ABSCANC question

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commscanaus

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I have just ordered an Antenex ABSCANC 150/450/800Mhz mobile antenna from Ameradio.com along with the optional spring. It will be installed on the luggage racks of my new car. Yes, I know I should just man up and drill the centre of the roof, but I just cannot bring myself to do it!

Does the ABSCANC need a decent ground to work effectively?
The luggage racks are grounded to the car body, bu the antenna will be mounted over the roof and not directly to it.
I could also run into problems with the antenna striking objects overhead as I park underground for work and in the garage at home with a tilitng door. I am hoping that the Antenex will be strong enough and that the spring option will help ease any impacts.

On the previous vehicle, I used trunk lip mounts which worked well. Changing antennae all the time got old very quickly which is why I ordered the Antenex.

It will be used on a Uniden BCD996T which listens to a P25 trunk system on 420Mhz, several type II systems and EDACS at 800Mhz and conventional analogue at 160Mhz.

Anyone have much success with the ABSCANC?

Commscanaus.
 

benbenrf

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Does it need a “ground”, or does it need a “ground-plane”?

Mounting the antenna with a thru-hole through the centre of the roof both grounds the antenna as well as provide as equi-spread a ground-plane as is possible to obtain on a car (short of then adding radials that stick out both sides of the car as far as the front and back stick out!).

In reality the performance difference you will experience (primarily in a receive-only scenario, but to a lessor extent also in a Tx scenario) with the antenna mounted in the centre of the roof (thru-hole) versus mounted on one or other side of the car to a roof rack (or bumper), will probably be measurable with lab instruments, but is going to have negligable effect in day to day use.

The variation in enviromental conditions your car/antenna set-up is going to experience, day to day as you drive around in it (i.e. variation in multi-path refelections off buildings, position of hills, changes in ground moisture & conductivity conditions, changes in relative position to contour lines, position of large volume steel mass objects - large steel framed buildings, car parks full of cars, blah blah, blah blah …...) is going to reduce to zero any advantage that could have been gained by way of mounting the antenna with a thru-hole in the centre of the roof.

That deals with ground-plane effect on antenna performance in the circumstances you are citing - and I emphasize in the circumstances you are describing.

But, from a pur grounding perspective i.e. bringing all parts in your radio/scanner set-up to a common and equal voltage potential - yes, as good a ground as is possible to establish is always the way to go – …. and then the subject starts getting complicated.
But your question really is: how is it going to impact the antenna performance?

Well, the antenna is going to be “grounded” in any event - thru the shroud/outer shield of the coax cable to the the receiver/scanner, which in turn is then “grounded” to the vehicle chassis by way of the negative (-) wire of the power supply that goes to the radio/receiver/scanner. So, in theory it is grounded.

Now - how technical do we want to get, and how effective is the above ground?
I have no experience with the antenna you are describing, but from what I can see on the net (i.e. pictures of it), the connection options offered with the example I looked at, would ground the antenna to the car at the fixture point. How effective that "ground" would be, if it was thru-hole, would be all about bare metal to bare metal contact (i.e. paint layers removed from between the antenna fixture and the panel it was been mounted to?), or how well any roof-rack was fixed to the car body (i.e. paint players between the roof and the rack fixture points) (?)

Will making an effort to deliberately isolate the antenna from “ground” at these points result in any descernable difference in performance?

mmmm …..... I see a big debate coming, so I'll answer it like this. Why make any effort to deliberately isolate it from ground? I tell you where it could result in a BIG difference (potentialy): in a lightening strike. Instead of the strike flowing around the outside of the car, on the body panels (Faraday Cage effect), it will now take the path of least resistance - down the coax to your radio!!!!!

In short: yes - a good earth is always a good idea - for performance reasons (if only theoretical, as often is the case) as well as safety and general common sense reasons.

As for it striking obstacles – well, most antenna manufacturers take that into account in ensuring that the spring located at the base of the antenna is sufficiently flexible to account for the torque of an antenna strike, and if I recall correctly the rule we used (years ago when I worked on mobile antenna design)was: so long as the contact/strike point on the antenna was no lower than around 50% or 66% of the length of the antenna (from the top), then the spring would bend before your bodywork! But you are going to be best to check that out when you get your antenna – don't take my word for it please.
 
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commscanaus

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Thanks for such a detailed answer benbenrf!

That has been a great help and also very timely, as I have added photos of the install into the shack pictures section, having recently finished the antenna system on the car.

Strangely enough, I had an earthing issue which caused very poor VHF reception. Many repeaters that were nice and strong in the old car setup were now quite weak and noisy. The luggage racks were found not to be earthed the the car roof through the mounts. I made up some small earth straps to electrically bond the aluminum rack to the steel pins that contact the sheet metal of the car roof. Presto! VHF reception was noticeably better. I have yet to test VSWR with the transceiver yet.
So it seemed that the antenna was floating above the cars ground potential and did not work very well until bonded properly to the car. Both radios are indeed earthed to the car ground via the battery.

The antenna struck one of those "maximum height" steel bar thingys at an underground carpark last week and survived just fine. Very pleased I ordered the optional spring!

OBXJeepGuy- The ABSCANC took forever to arrive, and I had just about given up when the package showed up 3 months after ordering it. I needed a tri band antenna, and no-one stocks such a thing locally. It performs quite well, although I find it a bit lacking at 800Mhz. Most of my listening is P25 trunking on 420Mhz, where it works very well. VHF airband seems to work well too.
If not going into the garage or underground parking lot, I can use a high gain 800Mhz antenna which is a 5/8 over a 5/8 (Antenex) and is outstanding.
For all around work and a reasonably low profile, the ABSCANC is a good compromise workhorse.

Commscanaus.
 
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